Nascent tar formation during polyvinylchloride (PVC) pyrolysis

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1 Available online at Proceedings of the Combustion Institute 34 (2013) Proceedings of the Combustion Institute Nascent tar formation during polyvinylchloride (PVC) pyrolysis Ben Gui, Yu Qiao, Dan Wan, Shuai Liu, Zainan Han, Hong Yao, Minghou Xu State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan , China Available online 16 September 2012 Abstract Pyrolysis experiments of polyvinylchloride (PVC) were performed to investigate the effects of peak temperature, holding time, and heating rate on the formation of nascent tar. The nascent tar samples were collected using a wire-mesh reactor where the secondary reactions of the evolved volatiles were minimized. The small compounds, such as benzenes and alkanes, were not detected in nascent tar in wire-mesh reactor, whose components are quite different from those of other tars in tube type reactor and vacuum reactor. At a heating rate of 1000 K/s, the quasi-3 rings and 3 rings group aromatics were the major components in nascent tar; while the content of 2 rings group aromatics increased from 7.02% to 31.75% with increasing peak temperature from 500 to 800 C. At a longer holding time of 300 s, an increase of 2 rings group aromatics from 7.02% to 50.33% was also observed for the nascent tar at 500 C, indicating that the tar composition significantly changed at different stages of PVC pyrolysis. It seems that 3 4 rings compounds form in the early stage and then 2 rings compounds release in the later stage of PVC pyrolysis. Based on the experimental results in this work, a new four-stage mechanism, including (1) dechlorination accompanied with inner cyclization, (2) aromatic chain scission, (3) release of quasi-3 rings or 3 rings group, and (4) release of 2 rings group, of the PVC tar formation was proposed. Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Polyvinylchloride (PVC); Pyrolysis; Nascent tar; Wire-mesh reactor; Secondary reaction 1. Introduction Municipal solid waste (MSW), especially plastics, can be considered as an important source of energy since they are inexpensive and easily to obtain liquid fuels and volatile products [1 4]. Currently there are a number of technologies to deal with waste plastics but each technology has its own advantages and disadvantages [5,6]. For Corresponding authors. Fax: addresses: yuqiao@mail.hust.edu.cn (Y. Qiao), mhxu@mail.hust.edu.cn (M. Xu). example, landfill has a lower cost but is limited by strict laws; while incineration could save the lands but the low heating value of MSW and large amount of noxious products during the process cause big challenges to this technology. Among those available technologies, the thermal recycling by pyrolysis is likely to be a promising technology for environmentally-friendly utilization of waste plastics [7 9]. However, the presence of polyvinylchloride (PVC) in the waste may cause a recycling problem when thermal treatment is involved [10 12]. Many papers have provided some basic understanding about the pyrolysis behavior of PVC /$ - see front matter Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

2 2322 B. Gui et al. / Proceedings of the Combustion Institute 34 (2013) under various conditions [13,14]. The two degradation stages of dechlorination and hydrocarbon formation have been widely recognized during PVC pyrolysis [4,14]. Cyclization reactions probably play a major role in arresting polyene propagation during the whole degradation process [15,16]. However, the detailed aromatic composition of the PVC tar reported in much of the literature was quite confusing [1,2,4]. For example, the total aromatics (mainly benzene group) in PVC pyrolysis tar observed by McNeill et al. [4] and Miranda et al. [2] were wt.% at 500 C and wt.% at 520 C, respectively. However, there are still some C 1 C 13 aliphatic hydrocarbons and chlorinated hydrocarbons in the tars [2,14]. Starnes and co-workers proposed a six-center concerted mechanism [15,17] on the polyene growth of PVC degradation in their earlier studies. However, the existence of chlorinated hydrocarbons and other alkanes in PVC pyrolysis tar presents a further objection to the mechanism [15]. The effects of formation of chlorinated hydrocarbons during PVC degradation are not clearly known. Hinz and Gerrard believed that the presence of secondary reaction is a significant factor influencing the chlorinated hydrocarbons formation [2,18]. However, 5.19 wt.% aliphatic hydrocarbons and 1.75 wt.% chlorinated hydrocarbons in pyrolysis tar were both concluded as the by-products rather than due to the effect of secondary reaction during the aromatization and cyclization by McNeill et al. [4]. Moreover, the detailed mechanism of intermediate transformation and polyene termination during cyclization and final scission processes remains unknown yet. Thermo gravimetric analysis (TGA), fluidized bed, fixed-bed reactor, and vacuum reactor are the most common apparatus used to study PVC pyrolysis [12,13,19]. Although the TGA techniques have been widely employed to determine the pyrolysis behavior of PVC, they were operated under the conditions (e.g., sample stacking and very slow heating rates) very different from those in industrial facility. On the other hand, it is not possible to determine or control the effect of secondary reaction on tar formation during PVC degradation in those reactors. Actually, the tars collected in those reactors are terminal rather than nascent. Little is known on the properties of nascent tar from PVC pyrolysis. As will be described in more details below, the wire-mesh reactor features a minimized inter-particle secondary reaction of the volatiles released and a relatively accurate control of particle time temperature history. Therefore, the effects of the secondary reaction on the tar formation are effectively minimized. To gain a fundamental understanding of PVC pyrolysis, there are clear needs to study the dechlorination and hydrocarbon formation in the absence of secondary reaction. This study aims to evaluate the influences of peak temperature, heating rate, and holding time on the formation of nascent tar during PVC pyrolysis. 2. Experimental 2.1. Sample preparation and tar collection Samples of commercial PVC plastic with a particle size between 106 and 150 lm were used in this study. Ultimate analysis shows that the sample contains 38.8% (wt.) carbon, 6.9% hydrogen, 50.3% chlorine with the remaining 4.0% not identified. Nitrogen and sulfur were also not detected in the analysis. A wire-mesh reactor similar to that described by Gibbins and co-workers [20,21] and Li and co-workers [22,23] was used in this study. As shown schematically in Fig. 1(a), approximately 8 mg of PVC sample was distributed within a round working area of 20 mm diameter between two layers of the wire-mesh. The distribution of PVC particles approached almost as same as that of single particles: the particles certainly existed as a single layer and did not come into significant contact with each other. The mesh was then heated up with an electrical current from room temperature to peak temperature at a heating rate of 10 or 1000 K/s and held for a desired time. The reaction gas (N 2, purity > %) passed through the sample-laden mesh and cooled down the PVC particles directly at a linear gas velocity of 0.1 m/s (4 L/min) when measured under ambient conditions. The nitrogen gas remained at room temperature before it met the heated mesh. The volatiles were rapidly swept away by this stream of nitrogen gas from the parent PVC particles as soon as they were released from the particles. The gas flow rate of 4 L/min used in this work was chosen based on the previous work [20 24] to ensure that the evolved volatiles did not flash back on the hot mesh. Too high a flow rate would cause unacceptable temperature distribution across the mesh. Therefore, the inter-particle secondary reactions of the evolved volatiles were minimized. It should be noted that, the use of a relatively high gas velocity passing through the PVC-particle-laden mesh also means that not all gases would be heated up to high temperature: a significant portion of the gas would pass through the mesh without being heated up [20 23]. The volatiles, once evolved from the particles, would be heavily diluted and quickly quenched by ice water mixture while being carried away from the parent PVC particles. The secondary reaction of volatiles/tars was further minimized. Therefore, the tar collected by Teflon in trap was named as nascent tar and recovered by washing the tar trap with acetone [25]. The tar yield was determined and measured on a weight basis, and the

3 B. Gui et al. / Proceedings of the Combustion Institute 34 (2013) Fig. 1. Schematic diagrams of (a) the wire-mesh reactor and (b) the tube type reactor. tar was defined as the residue of collected liquid sample after solvent evaporation. To gain a better understanding of the effects of secondary reaction on the tar formation, PVC pyrolysis experiment was also carried out in a tube type reactor with a length of 1200 mm and an internal diameter of 20 mm. As shown schematically in Fig. 1(b), the nitrogen gas with a linear gas velocity of 0.1 m/s (4 L/min, measured under ambient conditions) was injected from the reactor bottom and then heated up by the furnace. The PVC particles hold in a quartz basket were rapidly put into the tube reactor by an iron hook when temperature of reaction zone was heated up to the peak temperature. The temperature of the PVC particles was measured by a K type thermocouple embedded at the center of the particles. The heating rate of the PVC particles in the tube burner calculated from the readings of the thermocouple is about K/s, which is on the similar scale between 10 1 and 10 3 K/s of the wire-mesh reactor. The volatiles of PVC were then collected by Teflon in trap, which was put into an ice water condenser. The tar recovery procedure in the tube type reactor is the same as that in the wire-mesh reactor. The experiments in the tube reactor were carried out using 100 mg rather than 8 mg of PVC particles, which were distributed within a working area of 10 mm diameter (instead of 20 mm in the wire-mesh reactor). Therefore, the inter-particle secondary reaction in the tube type reactor is certainly more significant than that in the wire-mesh reactor. At the same time, the secondary reaction of volatiles/tars in the tube reactor was also more significant than that in the wire-mesh reactor due to the lack of flow gas employed as the diluent medium Tar characterization Each tar yield shown in this work represents the average yield of 20 repeats under the identical condition. Then, the tar obtained in each repeat was mixed together to meet the minimum volume required by GC MS and was characterized using an Agilent GC MS (7890 series GC with a 5975C MS detector) with a capillary column (HP5; length, 30 m; internal diameter, 0.25 mm; film thickness, 0.25 lm). Therefore, the compositions of the mixed tars under each condition were reported. Helium was used as carrier gas with the flow rate of about 1 ml/min. Injection was made in a split less mode at 290 C. The column temperature was programmed from 35 C (held, for 5 min) to 210 C, then to 290 C at 10 K/s. The MS acquisition occurred after 5 min solvent delay. The identification of the peaks in the chromatogram was based on the comparison with the standard spectra of compounds in the NIST library. After each analysis, all of these peak areas were measured to calculate the relative composition of tar. In order to gain a fundamental understanding of PVC tar formation, over 100 aromatic compounds were further classified into 6 groups in this work, including chlorinates hydrocarbons, 2 rings group, quasi-3 rings group, 4 rings group, quasi-4 rings group, 4 rings group, and others. The classi-

4 Table 1 The structure of main aromatics in the nascent tar. Chlorinated hydrocarbons 2 rings group Quasi-3 rings group 3 rings group Quasi-4 rings group 4 rings group 2324 B. Gui et al. / Proceedings of the Combustion Institute 34 (2013)

5 B. Gui et al. / Proceedings of the Combustion Institute 34 (2013) fication and the main compounds of each group were shown in Table Results and discussion 3.1. Comparisons of PVC tars in different reactors A comparison of pyrolysis tars obtained in different reactors at the same peak temperature of 500 C was shown in Table 2. It can be found that the components of these tars were significantly different although the formation of aromatic compounds still played a principal role during the PVC pyrolysis. The literature [4] suggests that when mg PVC samples were heated for 20 min under a low pressure (10 5 mm Hg) in a vacuum reactor, as shown in Table 2, 88.20% of the aromatic compound (mainly benzene) was determined in the liquid products. A benzene formation mechanism was proposed based on the presence of benzene in pyrolysis tar during the PVC degradation [4]. However, the benzene compound of tar collected in a tube type reactor was only 18.70%, which is much lower than that in the vacuum reactor. Furthermore, 45.17% condensed ring aromatic of tar, including naphthalene, acenaphthene, fluorene, anthracene, pyrene, and other substituted aromatics, was observed compared with that of 2.4% in the vacuum reactor. The PVC tars were also collected in a wiremesh reactor at the heating rate of 10 and 1000 K/s to peak temperature of 500 C and held for a period of 300 s. An interesting observation arises from the comparison among different reactors. The data in Table 2 show that the amount of composition of tars collected by the wire-mesh reactor is much less than other tars obtained by the vacuum reactor and tube type reactor. Neither benzene nor alkyl aromatics were measured in the tars collected by the wire-mesh reactor, where the secondary reactions of the evolved volatiles were minimized. It was indicated that the minimum of the secondary reaction causes more pure tar. On the other hand, the concentrations of condensed ring aromatics in pyrolysis tar collected by the wire-mesh reactor shown in Table 2 were 77.98% at 10 K/s and 92.80% at 1000 K/s, respectively. It is believed that condensed ring aromatics were the main components of nascent tars during PVC pyrolysis in the wire-mesh reactor. As the secondary reaction is a significant factor influencing the tar compositions, the less content of the condensed ring aromatics in the vacuum reactor compared with that in the wire-mesh reactor is most likely due to the cyclization from the condensed ring aromatics to benzene group during volatiles released at the presence of secondary reaction between the volatiles and particles Nascent tar formation during PVC pyrolysis Effect of temperature on nascent tar formation Experiments were carried out by using a wiremesh reactor at a heating rate of 1000 K/s without holding at various temperatures from 500 to 800 C. Figure 2(a) shows the tar yield as a function of temperature during the pyrolysis of PVC. The average tar yield increases from 6.13 wt.% at 500 C to wt.% at 800 C. The t-test demonstrated that the increase observed was statistically significant with a confidence of more than 95%. The low tar yield at low pyrolysis temperature may lead to the difficulty in detecting the composi- Table 2 The composition of the tars which collected by different reactors. Vacuum reactor, area% a Tube furnace, area% Wire-mesh reactor, area% Conditions Compound Temperature 500 C 500 C 500 C 500 C Heating rate 10 K/min K/s 1000 K/s 10 K/s Holding time 20 mins 20 mins 300 s 300 s Aromatic Benzene Alkyl aromatic Alkenyl aromatic Condensed ring aromatic Alkanes Cycloalkanes Alkenes Cycloalkenes Chlorinates hydrocarbons Others Total a Data were taken from McNeill et al. [4].

6 2326 B. Gui et al. / Proceedings of the Combustion Institute 34 (2013) Fig. 2. Tar yields during PVC pyrolysis (a) at a heating rate of 1000 K/s from 500 to 800 C with non-holding time; (b) at a heating rate of 1000 K/s to 500 C with the holding time from 0 to 300 s; (c) under different heating rates (10 and 1000 K/s) at the temperature of 500 and 800 C with non-holding time. The error bars indicate one standard deviation. Note that the scale of x-axis in panels (b) and (c) is not linear. tions of tar using a GC MS. As shown in Fig. 3(a), the nascent tar collected at 500 C contains 3.94% 4 rings, 7.67% quasi-4 rings, 46.33% 3 rings, 35.04% quasi-3 rings, and 7.02% 2 rings groups, and has similar compositions as the tar at 600 C. It can be found that the compounds in the nascent tar at a relatively lower temperature exist in two main groups: 3 rings group and quasi-3 rings group. However, the data in Fig. 3(a) show a significant change in compositions of the tar at 700 C, which contains 3.38% 4 rings, 5.60% quasi-4 rings, 24.94% 3 rings, 29.78% quasi-3 rings, and 31.75% 2 rings groups. Clearly a significantly increasing formation of 2 rings group was found with increasing temperature from 600 to 800 C. On the other hand, 0.64% and 1.68% of chlorinates hydrocarbons were measured in tars at 700 and 800 C, respectively, indicating that a small amount of residual chlorine remained in char and then released as an organic form at a relatively higher temperature [2], although the main dechlorination process at lower temperature (below 500 C) is well known Effect of holding time on nascent tar formation Pyrolysis experiments were also carried out by using a wire-mesh reactor at a heating rate of 1000 K/s with the holding time of 0, 30, 100, and 300 s. Figure 2(b) indicates that the tar yield increases significantly with the holding time during the PVC pyrolysis at a heating rate of 1000 K/s at 500 C. An average tar yield of wt.% was obtained with a 300 s holding time, compared with 6.13 wt.% without holding. Figure 3(b) shows that there are still no chlorinate hydrocarbons in the nascent tar after a holding time of 300 s at 500 C. As was explained

7 B. Gui et al. / Proceedings of the Combustion Institute 34 (2013) Fig. 3. Tar compositions classified into 6 groups during PVC pyrolysis (a) at a heating rate of 1000 K/s from 500 to 800 C with non-holding time; (b) at a heating rate of 1000 K/s to 500 C with the holding time from 0 to 300 s; (c) under different heating rates (10 and 1000 K/s) at the temperature of 500 and 800 C with non-holding time. The compositions of tar were classified into 6 groups. Note that the scale of x-axis in panels (b) and (c) is not linear. above, the volatilization of residual chlorine during the secondary stage of PVC pyrolysis is greatly affected by temperature. At a relatively low temperatures of 500 C and a fast heating rate, the effect of holding time on formation of chlorinates hydrocarbons is not significant. The data in Fig. 3(b) also shows that the concentrations of 2 rings group in tar increased from 7.02% to 50.33% after holding a period of 300 s at 500 C. Meanwhile, the percentage of 3 rings and 4 rings groups in nascent tar decreased from 46.33% and 3.94% to 23.40% and 2.05%, respectively, due to much more formation of 2 rings group with the longer holding time. This clearly shows a significant change of tar compositions at different stages of PVC pyrolysis. The data in Fig. 3(b) further confirm that the release of large aromatic ring systems with 3 4 fused benzene rings took place in the early stage of hydrocarbon formation. There are at least two possible mechanisms for benzene aromatic formation [4,16]. One is direct cyclization by alkenes, and the other is based on polycyclic aromatic scission. In this work, the formation of 2 rings compounds in the later stage is most likely due to the scission of polycyclic bonds rather than the direct formation by alkenes during the pyrolysis of PVC Effect of heating rate on nascent tar formation In order to gain a better understanding of the effect of heating rate on nascent tar formation, the PVC samples were pyrolyzed at 500 and 800 C with a fast heating rate of 1000 K/s and a slow heating rate of 10 K/s. Figure 2(c) shows the effect of heating rate on tar yield during PVC pyrolysis. It shows that the increase of heating rate will lead to the increase of the tar yield at the same pyrolysis temperature. For example, the tar yield at 800 C increased from to

8 2328 B. Gui et al. / Proceedings of the Combustion Institute 34 (2013) wt.% when the heating rate was increased from 10 to 1000 K/s, while the tar yield at 500 C increased from 4.74 to 6.13 wt.% for the same increase of the heating rate. It also can be found that the effect of heating rate becomes more significant at higher pyrolysis temperature. As shown in Fig. 3(c), the concentration of 2 rings compounds in nascent tar collected at 500 C sharply decreased from 20.35% to 7.02% when the heating rate was increased from 10 to 1000 K/s. As was explained above, the release of small aromatic ring systems with 2 benzene rings occurs in later stage of hydrocarbon formation. The more formation of 2 rings compounds at 10 K/s heating rate is most likely due to the longer reaction time before reaching the peak temperature. The formation of 2 rings compounds correlates with the reaction time: the longer the reaction time, the more production of the 2 aromatic ring systems. The similar trends were also observed for the tars collected at 800 C. Figure 3(c) also shows that the concentration of large molecule in nascent tar collected at 500 C, especially quasi-3 rings and 3 rings groups, significantly increased as the heating rate changed from 10 to 1000 K/s, although the increasing heating rate resulted in only a slight increase in tar yield at 500 C. It was clear that the fast heating rate could cause large molecule formation. Another very interesting observation in Fig. 3(c) arises from the comparison of chlorine compound in nascent tars at 500 C between the cases of using the fast heating rate and the slow heating rate. As explained above (Fig. 3(b)), no chlorinates hydrocarbons in the nascent tar were detected at 500 C even after a holding time of 300 s. The data in Fig. 3(c) indicates that the slow heating rate of 10 K/s would cause more release of organic chlorine compounds from residual char at a relatively lower temperature of 500 C. The data in Fig. 3(c) also show that the higher peak temperature can improve the release of residual chlorine Discussions on reaction mechanism As was stated above, there are at least two possible mechanisms for benzene aromatic formation between direct cyclization and polycyclic aromatic scission. In the absence of significant secondary reaction, the formation and release of nascent tar during PVC pyrolysis has been well understood in this study. The polyene termination is most likely due to a scission procedure of polycyclic bonds, which is somewhat similar to the mechanism proposed by Jordan and his co-workers [16]. In a previous work [16], based on the analysis using TGA, a degradation mechanism of four regions was concluded on the pyrolysis of de- HCl PVC: (1) final stage of de-hcl; (2) random chain scission with subsequent recombination; (3) cyclization/aromatization; and (4) degradation from coke formation. In fact, polycyclic structure was probably formed before tar release in a wiremesh reactor. With increasing temperature or Fig. 4. Proposed mechanism of nascent PVC tar formation.

9 B. Gui et al. / Proceedings of the Combustion Institute 34 (2013) holding time, the PVC char degraded as aromatic chain scissions accompanied with crosslinked reforming. When the scission process came to 3 4 rings compounds, the tar began to release. The nascent tar released mainly as quasi-3 rings or 3 rings compounds in early stage, and then 2 rings compounds played a significant role in later stage. Therefore, a new four-stage mechanism of the PVC tar formation was proposed in this work, where the four stages are (1) dechlorination accompanied with inner cyclization; (2) aromatic chain scission; (3) release of quasi-3 rings or 3 rings group; and (4) release of 2 rings group. The schematic diagram of the proposed mechanism was shown in Fig Conclusion The effects of peak temperature, holding time, and heating rate on nascent tar formation during PVC pyrolysis were investigated in a wire-mesh reactor. The following main conclusion can be drawn: (1) The secondary reaction of volatiles/tars can significantly affect tar formation during PVC pyrolysis. The small compounds, such as benzene group and alkanes, were not detected in nascent tar collected by the wire-mesh reactor compared with those collected by a tube type reactor and a vacuum reactor. (2) As temperature increasing from 500 to 800 C, tars released as 3 4 rings compounds in the early stage of hydrocarbon formation, and then released as 2 3 rings compounds in the later stage. The same process was observed when increasing the holding time from 0 to 300 s or decreasing the heating rate from 1000 to 10 K/s. (3) A new four-stage mechanism of the PVC tar formation was proposed: (1) dechlorination accompanied with inner cyclization; (2) aromatic chain scission; (3) release of quasi- 3 rings or 3 rings; and (4) release of 2 rings group. (4) A small amount (less than 1%, based on the total chlorine in PVC) of chlorine remained in char after de-hcl process and finally released as an organic form at relatively lower heating rate or higher temperature. Acknowledgements The authors gratefully acknowledge the support of this study by the National Basic Research Program of China (2011CB201505) and the National Natural Science Foundation of China ( , and ). The authors also gratefully thank the discussions with Dr. Yun Yu at the Curtin University in Perth, Western Australia, Australia. The support of the Analytical and Testing Center at the Huazhong University of Science and Technology, Wuhan, China, is also appreciated. References [1] I.C. McNeill, L. Memetea, Polym. Degrad. Stab. 43 (1) (1994) [2] R. Miranda, H. Pakdel, C. Roy, H. Darmstadt, C. Vasile, Polym. Degrad. Stab. 66 (1) (1999) [3] R. Miranda, H. Pakdel, C. Roy, C. Vasile, Polym. Degrad. Stab. 73 (1) (2001) [4] I.C. McNeill, L. Memetea, W.J. Cole, Polym. Degrad. Stab. 49 (1) (1995) [5] W. Kaminsky, J.S. Kim, J. Anal. Appl. Pyrolysis 51 (1 2) (1999) [6] H. Kastner, W. Kaminsky, Hydrocarbon Process. 74 (5) (1995). [7] W.H. Starnes, B. Du, S. Kim, V.G. Zaikov, X. Ge, E.K. Culyba, Thermochim. Acta 442 (1 2) (2006) [8] C.H. Wu, C.Y. Chang, J.L. Hor, S.M. Shih, L.W. Chen, F.W. Chang, Waste Manage. (Oxford) 13 (3) (1993) [9] S. Kim, Waste Manage. (Oxford) 21 (7) (2001) [10] M. Bisi, C. Nicolella, E. Palazzi, M. Rovatti, G. Ferraiolo, Chem. Eng. Technol. 17 (1) (1994) [11] H. Bockhorn, A. Hornung, U. Hornung, P. Jakobstroer, M. Kraus, J. Anal. Appl. Pyrolysis 49 (1 2) (1999) [12] R. Miranda, J. Yang, C. Roy, C. Vasile, Polym. Degrad. Stab. 72 (3) (2001) [13] R. Miranda, J. Yang, C. Roy, C. Vasile, Polym. Degrad. Stab. 64 (1) (1999) [14] C.H. Wu, C.Y. Chang, J.L. Hor, S.M. Shih, L.W. Chen, F.W. Chang, Can. J. Chem. Eng. 72 (4) (1994) [15] W. Starnes, Prog. Polym. Sci. 27 (10) (2002) [16] K. Jordan, S. Suib, J. Koberstein, J. Phys. Chem. B 105 (16) (2001) [17] W.H. Starnes Jr., J.A. Wallach, H. Yao, Macromolecules 29 (23) (1996) [18] P.G.T. Fogg, W. Gerrard, Solubility of Gases in Liquids, Wiley, New York, [19] P.T. Williams, E.A. Williams, Energy Fuels 13 (1) (1999) [20] J. Gibbins, R. King, R. Wood, R. Kandiyoti, Rev. Sci. Instrum. 60 (6) (1989) [21] J. Gibbins, R. Kandiyoti, Energy Fuels 3 (6) (1989) [22] C.Z. Li, K.D. Bartle, R. Kandiyoti, Fuel 72 (11) (1993) [23] C.Z. Li, K.D. Bartle, R. Kandiyoti, Fuel 72 (1) (1993) [24] R.C. Messenböck, D.R. Dugwell, R. Kandiyoti, Energy Fuels 13 (1) (1999) [25] E.L. Wynder, E.A. Graham, A.B. Croninger, Cancer Res. 13 (12) (1953) 855.